Idling problem 1980 911SC

[Prisco]

Just had car restored and can't seem to get the car to idle correctly. At start up, it idles at 1100 rpm, once driven a bit, the idle diesels up to 2,000 rpm and will not come down. If driven on the highway, when exiting the idle is always up at 2,000rpm or better and does not come down. Any ideas on how to tackle this or what causes it? Thanks

[LWJ]

Did you try the decel valve? There are two vacuum motors on the passenger side of the top of the engine. One has a vacuum hose going to it. Try pulling this hose and plugging with a golf tee. I had similar symptoms on my 80 SC. Worked very well.
Larry

[longhornchris04]

It could be a myriad of CIS parts that are acting up.

1 - check for any vacuum leaks. A common tactic is to spray a bit of carb cleaner near the vac lines. If you have a vacuum leak, it will play havoc with the CIS fuel metering

2 - assuming there are no vacuum leaks, try adjusting the idle adjustment. There's a large flat-head screw on the driver's side of the rubber boot. FAIR WARNING: its possible to adjust this such that the warm-up cycle doesn't work right - which is a sign that something else is wrong

3 - WUR is screwed up... beyond my skill level, but if you run a forum search there's lots of posts out there.

Run a search with the key words CIS ADJUSTMENT and you should find plenty of help.

[Scott R]

Start with the basics, is the distributor working correctly? I had a hanging idle like this and it was sue to broken springs on my advance weights.

After that move on to system pressure checks, then a vacuum leak test. It should be something easy.

[Gunter]

Originally Posted by Prisco

Just had car restored and can't seem to get the car to idle correctly.

Restored meaning? If the engine was rebuild, go back to the builder and let them set the correct mixture and idle. On cold-start, the idle should be around 13-1400 RPM and once the AAR closes after a few minutes, settle at ~900.

At start up, it idles at 1100 rpm, once driven a bit, the idle diesels up to 2,000 rpm and will not come down. If driven on the highway, when exiting the idle is always up at 2,000rpm or better and does not come down. Any ideas on how to tackle this or what causes it? Thanks

What exactly was done to the engine when restored?

[DDD]

I vote for the decel valve. My '83 SC does the same thing, but it isn't consistent. Seems the colder it is outside the more likely it happens, especially after 10 mins on the highway. If I artifically lower the RPMs with the clutch, I can get the RPMs to drop some, and usually they stay down where I dropped them to. I've always assumed I had a decel valve problem based on lots of other stuff I've read, and have never tried to fix it since the problem isn't a big deal and is intermittent.

Let us know what you figure out.